Related knowledge about SQL DEFAULT constraints

SQL DEFAULT Constraints are very important in php, this article will explain them.

SQL DEFAULT constraint

DEFAULT constraint is used to insert a default value into a column.

If no other value is specified, the default value will be added to all new records.

SQL DEFAULT Constraint on CREATE TABLE

The following SQL creates a DEFAULT constraint for the "City" column when the "Persons" table is created:

My SQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
Id_P int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255) ,
Address varchar(255),
City varchar(255) DEFAULT 'Sandnes'
)

By using something like GETDATE() Function, DEFAULT constraints can also be used to insert system values:

CREATE TABLE Orders
(
Id_O int NOT NULL,
OrderNo int NOT NULL,
Id_P int,
OrderDate date DEFAULT GETDATE()
)

SQL DEFAULT Constraint on ALTER TABLE

If created for the "City" column when the table already exists DEFAULT constraint, please use the following SQL:

MySQL:

ALTER TABLE Persons
ALTER City SET DEFAULT 'SANDNES'

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'SANDNES'

Revoke DEFAULT constraint

If you need to revoke DEFAULT constraint, please use the following SQL:

MySQL:

ALTER TABLE Persons
ALTER City DROP DEFAULT

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